Masteries

Offense:
1

Defense:
21

Utility:
8

Introduction

Hey Guys. This is my first guide, and I just wanted you to see my build for Rammus, which is, for the note, completely defensive.
Also note, that this build is made by the presumption, that the enemy team is well mixed with both melee and AP champions, however I shall also describe some of the exeptions there may be.

Runes

Masteries

I find it a good idea, going 1/21/8.
Even though it may seem a bad idea because of the missing point in Utility, I find that getting the Exhaust "Upgrade" values more than earning that extra gold, that Greed gains you.

When already having given Exhaust it's upgrade, you might as well give Teleport it's upgrade as well in order to move on in the Mastery Tree. I will explain the importance of these two particular spells, in the following section.

Summoner Spells

Here, I have chosen Exhaust and Teleport.

Exhaust:
Exhaust is, in my eyes a must for Rammus, in order to be able to target people properly.
The deal is: You go Powerball and head on into a chase, you hit a target, you Taunt, and Shield right away before they get away, and hopefully your team will target that person. If you by mistake should get to someone which is rather bulky, or the team haven't caught up with you, you use Exhaust, to ensure the kill.

Teleport:
In my opinion, also a must for any tank-ish character. It is very important to be capable of initiating a fight, get away after taking most of the damage(Your Taunt should grant you a lot of attention on open lane team fights), recall, and then Teleport right back into the action. You may say that, this sort of doubles your efficiency in battle, as long as you survive after your first assault(Hopefully having killed an enemy or two). Your only hope would be that none of your teammates get too aggresive in the short period you are gone.

Items during Gameplay

First items to start off with, would be [*] Doran's Shield, and [*] Health Potion.

Your best lane mate would be either a healer as Soraka, or a early-game strong dps, who can help you get firstblood. You want to depend on whichever lane mate you end up with to be fast out of spawn, so you can get to the bushes on your lane ASAP. Best case scenario, you get to "their" bush, before they show up.
When your enemy are walking towards the bush, use Powerball, not moving. Just stay right there at the end of the bush(In front of your lane mate), until they walk into you. If any of the two Champions at your lane are squishy enough, Exhaust them, and try to kill them as fast as possible.Be advised, watch out with getting hit by long range skills, like Mundo's Infected Cleaver. If any Champion with such skill arives at your lane, attempt to avoid the attack, by going back in the bush, and then rush forward again with Powerball. This way they think nobody is there, and yet get severely surprised. This scenario depends on your lane mates capability of foreseeing this ranged attack into the bush.

After earning a good 850 gold, head back, to get your first main survival item, [*] Ninja Tabi. Head back to your lane after this. If you have money to spare get other items depending on the nature of the enemy team, like [*] Sight Ward (If they are looking to want to gank early, [*] Vision Ward (If there are any invisible champs), or just bring more potions to be able to stay at the lane longer.

Your next purchase is based on your lane opponents. If you are lucky enough to face two AD champions, and maybe, even better, melee ones, go straight for [*] Sunfire Cape, getting the [*] Giant's Belt before [*]Chain Vest, since there's a chance you might be ganked by AP champion's from other lanes at this time.
Are you on the other hand against either to AP Champions, or one of each kind, you may want to build either [*] Banshee's Veil or [*] Force of Nature, depending on the amount of CC(Slow, Stun, etc.) the enemy team is in possession of. BV if they have a lot, as well as use it to harrass you, and FoN if you're having it a bit more easy, and want to be able to stay in the field more(This is allowed due to the health regen from FoN).

After getting either one of these three items, get the other one. If you got [*] Sunfire Cape first, unless the entire enemy team is AD-based, get either [*] Banshee's Veil or [*] Force of Nature afterwards. If the entire enemy team is indeed AD-based, replace BV/FoN with [*] Thornmail, for them to feel some further pain from focusing you.

Once you have both [*] Sunfire Cape and [*] Banshee's Veil / [*] Force of Nature, it's time to get some HP. [*] Warmog's Armor is the perfect solution for this, and if you chose to get Force of Nature as well, your health regen will be simply insane.

After having WA added to your items, if the game has not already ended, you should apply [*] Guardian Angel to your stock, in order to be capable of going a little more offensive. All you can hope for is that the enemy will still want to attack you, even though you have this item. Basically, if your team is in control at this moment it may slip the enemy's eyes that you are building this item, and if you earlier got [*] Banshee's Veil, the visual part of it, should be well covered in by the round veil from exactly Banshee's Veil. Hope they oversee it, take a load of damage, and have your team destroy them, while they wonder why they didn't get a kill when you went down.

If you, after getting these 6 items, are still not by the end of the game, and are losing, you might want to get the other items I mentioned along the way of this text.Meaning, if you got [*] Sunfire Cape and [*] Banshee's Veil, you should get [*] Thornmail or [*] Force of Nature, depending on who you find yourself taking the most damage from.

Summary

Overall, Rammus is an awesome tank, with little to fear, as long as his teammates avoid feeding anyone from the enemy team. If someone get's fed, or is starting to get fed, a gank by pretty much the whole team(if possible) should be done asap, in order to stop an eventual Stacking of Mejai's Soulstealer or of the alike.

If you play Rammus well enough, your opponents should hate you for it, and hopefully you will be able to win quite some games, as well as save some teammates with your Taunt.